With one aim of lamp and lighting designers being to achieve a high brightness light source which may be applied to an optical fiber light distribution scheme, there comes a commensurate need to develop a ballast arrangement which can efficiently power such a bright light source and can also exhibit advantages in the area of the size and cost considerations so as to be useful in limited space environments. The light source for a central lighting system which utilizes optical fibers for light transmission to various spaced apart locations must provide a high brightness output so as to insure that a sufficient amount of light output is available for coupling to the input face of the optical fibers. An example of such a high brightness light source can be found in U.S. Pat. No. 5,239,230, entitled "High Brightness Discharge Light Source" filed contemporaneously herewith in the name of Allen et al and which is assigned to the same assignee as the present invention. This application discusses a central lighting system which must provide the desired output in as small a space as possible. For instance, in an automotive application, the light source and the supporting circuitry to operate the light source must fit within a small area so as to insure aerodynamic advantages to the automotive designer. U.S. Pat. No. 4,958,263 issued to Davenport et al. on Sep. 18, 1990 and assigned to the same assignee as the present invention discusses an automotive application for a central lighting arrangement.
In addition to the size restrictions for a central lighting system and circuitry therefor as used in an automotive environment, it is necessary that such lighting system provide instantaneous light so that as the driver turns on the headlamps, light is instantaneously provided rather than after a warm-up period as in the case of the typical discharge light source. To this end, U.S. Pat. No. 5,059,865 issued to Bergman et al on Oct. 22, 1991 and issued to the same assignee as the present invention, illustrates the use of a xenon metal-halide lamp which provides instantaneous light output. It is required of the circuitry for such light source that the necessary energy to effect instantaneous light be provided; such required energy may be on the order of approximately 20 volts at 6 amps. Furthermore, in the event that the lamp is extinguished, the light output must be reestablished in a substantially instantaneous manner if desired. This restarting feature, typically referred to as a hot restrike operation, has been performed in the past using a conventional iron core transformer to achieve the high voltage input needed to restrike a discharge lamp that has not cooled from the previous on condition. It can be appreciated that in a central lighting system where it is desired to keep size requirements to a minimum, that a typical iron core transformer is both slow and oversized. Therefore, it would be advantageous for the ballast circuit for use with a discharge light source applied to a centralized lighting system, if such ballast had hot restrike capabilities that were achieved using components of a minimum size and wherein such components were capable of operating repeatedly in a rapid manner.
An example of a DC ballast arrangement for a xenon metal-halide lamp suitable for automotive applications can be found in U.S. Pat. No. 5,047,695 issued to Allen et al on Sep. 10, 1991 and assigned to the same assignee as the present invention. This patent discusses the adverse effects of cataphoresis to lamp operation when such lamp is disposed in a horizontal orientation. This patent presents a solution for avoiding such adverse effects by use of a ripple current imposed upon the operating signal which drives the lamp.
A central lighting system for an automotive application has a further requirement that, since the light source must operate from the automotive battery, in the event of a low voltage condition of the battery, the light source must not present a severe drain to the battery by drawing more current to maintain a constant level of light output. In such a low voltage condition, it would be advantageous of the light source ballast arrangement to sense the low voltage and then clamp the current at a fixed value such that the light output becomes dim providing an indication of the low voltage condition and also thereby preventing the more rapid discharge of the battery current. Though the conditions of preferred operation of the low voltage operation of the ballast circuit for a high brightness discharge light source have been described in relation to an automotive application, it can be appreciated that the principles of operation for such ballast arrangement apply in other environments as well such as in display lighting, lighting for medical instruments, and other lighting applications requiring a high brightness source in a small space.